Vertical cavity surface emitting lasers (VCSELs) have found numerous applications ranging from free space, plastic optical fiber, scanning, printing, machine vision, optical switching, displays, multi and single channel communication systems, and short-range fiber optical communication systems using plastic optical fibers (POF). These POFs have a local attenuation minimum at 650 nm and hence VCSELs emitting in this wavelength rage are important for realizing such systems. Most red emitting VCSELs are fabricated using traditional and expensive semiconductor growth techniques such as molecular beam epitaxy and metal organic chemical vapor deposition.
A quantum dot (QD) is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules. The conducting characteristics of quantum dots are closely related to the size and shape of the individual crystal making tip the quantum dot. Colloidal quantum dots are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The synthesis of colloidal quantum dots is based on a three-component system composed of: precursors, organic surfactants, and solvents. When heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into monomers. Once the monomers reach a high enough supersaturation level, the nanocrystal growth starts with a nucleation process, eventually producing the quantum dots. An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the nanocrystal's quantum confined size is more significant at energies near the band gap. Thus, because of the quantum confinement effect, quantum dots of the same material, but with different sizes, can emit light of different colors.
Embedding photon emitters such as QDs in microcavities alter their emission properties due to the ability of these structures to confine and enhance electromagnetic fields. Colloidal QDs have been embedded in distributed feedback structures, poly (methylmethacrylate) spheres, silica microspheres, one dimensional microcavities, two and three-dimensional photonic crystals and microdisk structures. Such structures are important for the realization of a compact laser due to their smaller footprint and decreased lasing threshold due to smaller optical mode volumes.
The simplest class of microcavities suitable for lasing is a one dimensional microcavity consisting of a cavity layer sandwiched between two sets of Distributed Bragg Reflectors (DBRs). A DBR is formed from alternating layers of materials with different refractive indices. Each layer has a uniform thickness, and each layer boundary causes a partial reflection of an optical wave. For waves whose wavelength is close to four times the optical thickness of the layers, the many reflections combine with constructive interference, and the layers act as a high-quality reflector. Most DBRs for VCSEL structures are fabricated using techniques such as MBE, MOCVD, plasma enhanced chemical vapor deposition, or sputtering. These methods of manufacturing disadvantageously require expensive manufacturing tools, and are slow to form layers.
In contrast, spin coating is a relatively fast and inexpensive method to spread a material, but conventional spin coating does not provide precise control over the thickness of deposited material. Spin coating has been used where variations in thickness are not critical, such as to form an emissive organic material onto Bragg mirrors and DFB structures. Optically and electrically pumped VCSEL devices have used solid substrates, with the exception of surface emitting lasers utilizing two-dimensional photonic crystal based reflectors with a thick (approximately 1 μm) organic dye based gain medium to realize a flexible laser structure.
Flexible microcavities have been demonstrated using a commercially available reflector film acting as the bottom mirror and a metal mirror as the top mirror. However, the use of metal mirror reduced the reflectivity drastically and hence does not allow the realization of high quality factor microcavities.
Light structures that generate visible radiation have been used to deliver photodynamic therapy. For example a light bandage has been created by combining off the shelf light emitting diodes (LEDs) and embedding them in an epoxy. A disadvantage of such devices is that because they operate in the visible part of the electromagnetic spectrum, they suffer poor effectiveness when used for some applications of the therapy, such as wound healing, which are more effective when used with electromagnetic radiation outside the visible range.